1. Field
One or more embodiments described herein relate to an organic light emitting diode display, a method of driving such a display, and a manufacturing method of manufacturing such a display.
2. Description of the Related Art
An organic light emitting display generates images based on light from organic light emitting diodes (OLEDs). This type of display has reduced power consumption and improved response speed, viewing angle, and contrast ratio compared with other display technologies. An OLED display is able to achieve this level of performance, at least in part, because OLEDs are self-emissive elements and therefore no additional light source (e.g., backlight) is required. Furthermore, OLED displays have reduced thickness and provide improved luminance and color purity. These types of displays are also flexible.
The pixels (or sub-pixels) of an OLED display emit primary colors such as red, blue, green, white, and the like. The light from these pixels (or sub-pixels) is combined to express a full range of colors from each pixel.
Each pixel (or sub-pixel) a pixel driving circuit for driving an OLED with a light emission current corresponding to a data signal. The OLED includes a light emission layer between a pixel electrode and an opposing electrode. The pixel and opposing electrodes serve as anode and cathode electrodes. In operation, electrons injected from the cathode and holes injected from the anode combine in the light emission layer to generate excitons. When the excitons change state, they release energy to emit light.
The opposing electrode may be formed across the pixels and may be applied with a fixed common voltage. In one example of the pixel driving circuit, the pixel driving circuit may include a driving transistor coupled to the OLED, at least one capacitor coupled to the driving transistor, and a switching transistor coupled to the driving transistor.
Additionally, the pixel driving circuit is coupled to a driving voltage terminal ELVDD. The switching transistor has a source terminal coupled to a data line and a gate terminal coupled to a gate line. The switching transistor is turned on when the gate signal from the gate line is a gate-on voltage, and is turned off when the gate signal is a gate-off voltage. When the switching transistor is turned on, the data signal transferred from the data line to the capacitor and the driving transistor.
The capacitor is coupled to the driving voltage ELVDD to hold the data signal for one frame. The capacitance of the capacitor serves to compensate a threshold voltage of the driving transistor, and thus has a great effect on light emission quality of the OLED display.
When the driving transistor is turned on by the data signal, the driving transistor adjusts the amount of current of the OLED on a path from the driving voltage ELVDD to the common voltage. The pixel emits light depending on the amount of current. In this example, the driving transistor is always turned on by the switching transistor and a data signal of a common connection terminal of the capacitor, to thereby continuously provide current to the OLED.
When the driving transistor for driving the OLED is driven for a long time, its threshold voltage or mobility varies. As a result, an expected luminance may not be generated. Particularly, when the characteristics of the semiconductor material in the driving transistors are not uniform throughout the display device, a luminance deviation may be generated among the pixels to degrade display quality.